Transistor amplifier circuit

ABSTRACT

In a transistor amplifier circuit having an amplifier transistor, a collector load resistance and an emitter resistance, a constant-current absorbing circuit is connected in parallel with the emitter resistance, and/or a constant current pressing-out circuit is connected in parallel with the collector load resistance, so that the distortion factor of the transistor amplifier circuit is lowered without spoiling the input dynamic range, the low supply voltage operation or the voltage amplification factor thereof.

United States Patent Kominami Aug. 12, 1975 TRANSISTOR AMPLIFIER CIRCUIT[75] Inventor: Yasuo Kominami, Kokubunji, Japan [73] Assignee: Hitachi,Ltd., Japan [22] Filed: Feb. 7, 1974 [2|] Appl. No.: 440,355

[30] Foreign Application Priority Data Feb. 7, i973 Japan 48-l4660 [52]US. Cl H 330/30 D; 330/18; 330/40 [51] Int. Cl. t. H03F 3/45; HO3F 3/68[58] Field of Search.,,.. 330/22, 30 D, 18, 28, 38 M, 330/40, 149

[56] References Cited UNITED STATES PATENTS 3,532,909 10/l970 Buckley330/40 X ll/l970 Solomon i. 330/38 M UX 12/l970 Lochstampfer 330/28 XPrimary Examiner.lames B. Mullins Attorney, Agent, or FirmCraig &Antonelli [57] ABSTRACT In a transistor amplifier circuit having anamplifier transistor, a collector load resistance and an emitterresistance, a constant-current absorbing circuit is con nected inparallel with the emitter resistance, and/or a constant currentpressing-out circuit is connected in parallel with the collector loadresistance, so that the distortion factor of the transistor amplifiercircuit is lowered without spoiling the input dynamic range, the lowsupply voltage operation or the voltage amplifica tion factor thereof.

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TRANSISTOR AMPLIFIER CIRCUIT BACKGROUND OF THE INVENTION 1. Field of theInvention The present invention relates to a transistor amplifiercircuit. More particularly, it is directed to a transistor amplifiercircuit having an improved distortion factor characteristic withoutemploying negative feedback.

2. Description of the Prior Art Acoustic equipment is generally requiredto have a low distortion factor. In a sound amplifier, for example,negative feedback, has hitherto been adopted for improvement of thedistortion factor characteristic. In some cases, however, it isimpossible to apply the negative feedback to the circuit. The presentinvention was motivated by the desire to reduce the distortion factor ofan FM multiplex semiconductor integrated circuit (hereunder abbreviatedto FM MPX IC") of the coincidence system type which has been deemed tobe the most excellent of demodulation systems for the FM stereophonicbroadcast, The input signal of the FM MPX IC, which is a compositesignal, is time-divided by a subcarrier of 38 KHz, and the dividedsignals are re spectively taken out to the right and left channeloutputs. It is, accordingly, meaningless in an FM MPX IC to subject theoutput signal to a negative feedback onto the input side, because theinput signal and the output signal are quite different.

The inventor therefore has analyzed the cause of the distortion of thetransistor circuit, and has developed an improvement which is intendedto render the distortion factor low without use of negative feedback.

Taking as an example a conventional groundedemitter transistor amplifiercircuit, such as shown in FIG. 5, let us consider the distortion factorof the amplifier circuit.

The AC equivalent circuit of the grounded-emitter transistor amplifiercircuit illustrated in FIG. 5 can be depicted as shown in FIG. 7(a).Now, when the resistances of bias resistors R, and R are sufficientlylarge, they are negligible, and the AC equivalent circuit shown in FIG.7(a) can be further simplified to one in FIG. 7(b). In the equivalentcircuit diagrams, i denotes the AC base current, 11;, the AC currentamplification factor, and r the emitter junction resistance. The emitterjunction resistance is a nonlinear resistance as understood from thebase emitter voltage (V emitter current (I,;) characteristic 1 in FIG.6.

The inventor considered that the emitter junction re sistance r,., beinga nonlinear resistance, would be the cause of the distortion of thetransistor amplifier circuit. The resistance r is evaluated as in thefollowing equation:

BE/ b Here, V is given from the diode rectification equation as follows:

From Equations (1) and (2).

r,- K T/(q E) In these equations, K denotes the Boltzmanns constant, Tthe absolute temperature, q the electronic charge, and I the emitterreverse saturation current.

Assuming that the TI-ID (distortion factor) of the circuit isproportional to the ratio of (the nonlinear input impedance)/(the totalimpedance), the distortion factor becomes:

From Equation (3),

TI-ID K T/(q I R On the other hand, the emitter resistance R as well asthe emitter current I and the base emitter voltage V have the followingrelation to the base DC bias voltage V,,:

V V,,,;+ R I therefor, I (V,, V J/R Substituting Equation (6) intoEquation (5),

(7) Accordingly, the distortion factor becomes independent of theemitter resistance R and the emitter current I,;, and depends on thetransistor base DC voltage V Therefore, in order to lower the distortionfactor of the transistor amplifier circuit shown in FIG. 5, the base DCvoltage V,, may be raised. When, however, the base DC voltage is raised,the emitter current I,,- increases, and the DC voltage drop across aload resistance R becomes large. As a result, the DC collector voltageof the transistor Q becomes prone to saturation, and the dynamic rangebecomes narrow.

In order to prevent this inconvenience, it is required to make thesupply voltage V high or to make the resistance of the load resistor Rlow. This is consistent with the generally known method for reducing thedistortion factor of any transistor amplifier circuit, and thehypothesis previously stated holds.

Thus, where it is intended to improve the distortion factorcharacteristic without utilizing negative feedback, the prior artsacrifices either the low supply voltage operation or the voltageamplification factor.

SUMMARY OF THE INVENTION It is therefore an object of the presentinvention to improve the distortion factor characteristic of atransistor amplifier circuit without employing negative feedback andwithout sacrificing the input dynamic range, the low supply voltageoperation or the voltage amplification factor. f

The fundamental construction of the present invention for accomplishingthe above object is characterized in that at least an amplifiertransistor, a collector load resistance and an emitter resistance areprovided, and that a constant-current circuit is connected in parallelwith the emitter resistance or the collector load resistance.

BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1 to 4 are schematic circuitdiagrams each showing an embodiment of the present invention, in whichFIG. 1 illustrates a grounded-emitter transistor amplifier circuit witha constant-current absorbing circuit added thereto;

FIG. 2 illustrates a grounded-emitter transistor amplifier circuit witha constant-current pressing-out circuit added thereto;

FIG. 3 illustrates a grounded-emitter transistor amplifier circuit witha constant-current absorbing circuit and the constant-currentpressing-out circuit added thereto;

FIG. 4 illustrates an FM multiplex circuit with a constant-currentabosrbing circuit and a constant-current pressing-out circuit addedthereto;

FIG. 5 is a schematic circuit diagram of a prior-art grounded-emittertransistor amplifier circuit;

FIG. 6 is a diagram of the emitter current (I )-base-emitter voltage (Vcharacteristic curve of a transistor circuit; and

FIGS. 7(a) and 7(b) are diagrams of the equivalent circuits of thegrounded-emitter transistor amplifier circuit as shown in FIG. 5.

PREFERRED EMBODIMENTS OF THE INVENTION EMBODIM ENT 1 As illustrated inFIG. 1, in a transistor amplifier circuit which includes an amplifiertransistor Q a collector load resistance R,,, an emitter resistance Rbias resistances R and R and an input capacitor C, and which has aninput terminal V, and an output terminal V,,, a constant-currentabsorbing circuit is connected in parallel with the emitter resistanceR,,-. The constantcurrent absorbing circuit is constructed of aconstantcurrent transistor Q and bias means consisting of diodes D and Dand resistances R and R and for causing a constant current to flowthrough the transistor Q With the above construction, theconstant-current absorbing circuit is added to the emitter resistance Rwhereby the constant current I flowing through the transistor Q of theconstant-current circuit bypasses the emitter resistance R Letting Idesignate the current flowing through the emitter resistance,

On the other hand, the base DC voltage V,, is given by:

n m; r: m: ar: RE s u) therefor I (R,,- I V inyl/ u Accordingly, fromEquation (5), the distortion factor becomes:

THD K T/Cq I,; R,

= (K T/q R ).IRy/IR|; lo (VB mall'l /I qI u 0+ a n1; Ill

Thus, at the same supply voltage V load resistance R and DC bias voltageV the circuit of the embodiment can attain a lowering of the distortionfactor as indicated in Equation l0) in comparison with the foregoingcase of Equation (7).

EMBODIMENT 2 As illustrated in FIG. 2, in the fundamental arrangement ofthe transistor amplifier circuit in Embodiment l, the current path of aconstant-current pressing-out circuit is connected in parallel with thecollector load resistance R of the amplifier circuit. Theconstantcurrent pressing-out circuit is constructed of aconstant-current transistor 0;, and bias means consisting of diodes Dand D and resistances R and R for causing a constant current to flowthrough the transistor Q With the above construction, the emittercurrent I becomes:

where I denotes the current flowing through the collector loadresistance R, and I the current of the constant-current pressing-outcircuit.

On the other hand, the distortion factor is given by Equation (7) asThus, even when the bias voltage V is raised in order to improve thedistortion factor THD and consequently the emitter current I;-increases, the rise of the voltage drop across the collector loadresistance R does not come into equestion because, owing to thetransistor 0 of the constant-current pressing-out circuit, the constantcurrent I bypasses the collector load resistance R to flow into theamplifier transistor Q,.

EMBODIMENT 3 As illustrated in FIG. 3, both the constant-currentabsorbing and pressing-out circuits according to the previous twoembodiments are added to the transistor amplifier circuit.

According to the present invention described above in connection withthe preferred embodiments, its object can be accomplished and its effectcan be brought forth on the ground stated below.

As previously explained, in order to improve the distortion factor ofthe transistor amplifier circuit without employing negative feedback,the emitter resistance R,,- may be made large, as apparent from Equation(4) (the base DC voltage may be increased as apparent from Equation (7)With the prior art circuit, however, the DC collector voltage of theamplifier transistor Q becomes easily saturated, and the dynamic rangebecomes narrow. In accordance with the present inven tion, in the caseof Embodiment I, even when the emitter resistance R is made large, thecurrent I flowing through the emitter resistance R is not made largeowing to the presence of the constant-current absorbing circuit.Therefore, neither the input dynamic range, the low supply voltageoperation nor the voltage amplification factor is sacrificed.

In the case of Embodiment 2, even when the base DC bias voltage V of theamplifier transistor Q, is made large to improve the distortion factor,the current I flowing through the collector load resistance R does notbecome large through the emitter current I,,- increases and owing to thepresence of the constantcurrent pressing-out circuit. Accordingly,within the input dynamic range. the low supply voltage operation nor thevoltage amplification factor is sacrificed.

Consequently. in the case of Embodiment 3, the DC bias voltage V can beraised, and therewith, the emitter resistance R, can be increased.Therefore, the distortion factor can be more improved withoutsacrificing the input dynamic range, the low supply voltage operation orthe voltage amplification factor.

The present invention can have the following aspects of performance inaddition to the following embodiments.

FIG. 4 shows an embodiment in which the present invention is applied toan FM MPX circuit. in the FM MPX circuit composed of differentialamplifier circuits, a constant-current pressing-out circuit (0 O Q R RR.) is connected in parallel with load resistances R and R for producingthe respective outputs (R L of right and left channels, while aconstantcurrent absorbing circuit (Q Q Q,,, R,) is connected in parallelwith an emitter resistance circuit (R R R The constant-currentpressing-out circuit constituted of the transistors Q 0 may also beconnected to the collectors of transistors Q1 and 0 Thus, for the samereasons as in the previous explanation, it becomes possible to realize alow distortion factor in signal circuits in which the negative feedbackhas been avoided, and the aforesaid object can be achieved.

As regards the constant-current absorbing circuit and theconstant-current pressing-out circuit. it is a matter of course that anyforms of the circuit arrangements can be adopted insofar as they aremeans to absorb and press out a substantially constant currentindependent of signals by bypassing the emitter resistance R and thecollector load resistance R, respectively.

The present invention is applicable to a transistor signal circuithaving at least an emitter resistance or a collector load resistance.

What is claimed is:

l. A transistor amplifier circuit comprising:

first to sixth transistors, each having an emitter, a

base and a collector;

a first resistor, one end of which is connected to the emitter of saidfirst transistor;

a second resistor, one end of which is connected to the emitter of saidsecond transistor;

a third resistor, one end of which is connected to ground;

means for connecting the other ends of said first to third resistors incommon;

means for connecting the emitters of said third and fourth transistorswith the collector of said first transistor;

means for connecting the emitters of said fifth and sixth transistorswith the collector of said second transistor;

means for connecting the base of the third transistor with the base ofthe sixth transistor;

means for connecting the base of the fourth transistor with the base ofthe fifth transistor;

a fourth resistor, one end of which is connected to the collectors ofsaid third and fifth transistors;

a fifth resistor, one end of which is connected to the collectors ofsaid fourth and sixth transistors;

means for connecting the other ends of said fourth and sixth resistorswith a first voltage source;

means for connecting the base of said first transistor with a secondvoltage source;

means for connecting the bases of said fourth and fifth transistors witha third voltage source;

first input means for applying a first input signal to the base of saidsixth transistor;

second input means for applying a second input signal to the base ofsaid second transistor;

a first constant current circuit connected between the emitter of saidfirst transistor and ground;

a second constant current circuit connected between the emitter of saidsecond transistor and ground;

a third constant current circuit connected between the collector of saidfirst transistor and said first voltage source; and

a fourth constant current circuit connected between the collector ofsaid second transistor and said first voltage source,

1. A transistor amplifier circuit comprising: first to sixthtransistors, each having an emitter, a base and a collector; a firstresistor, one end of which is connected to the emitter of said firsttransistor; a second resistor, one end of which is connected to theemitter of said second transistor; a third resistor, one end of which isconnected to ground; means for connecting the other ends of said firstto third resistors in common; means for connecting the emitters of saidthird and fourth transistors with the collector of said firsttransistor; means for connecting the emitters of said fifth and sixthtransistors with the collector of said second transistor; means forconnecting the base of the third transistor with the base of the sixthtransistor; means for connecting the base of the fourth transistor withthe base of the fifth transistor; a fourth resistor, one end of which isconnected to the collectors of said third and fifth transistors; a fifthresistor, one end of which is connected to the collectors of said fourthand sixth transistors; means for connecting the other ends of saidfourth and sixth resistors with a first voltage source; means forconnecting the base of said first transistor with a second voltagesource; means for connecting the bases of said fourth and fifthtransistors with a third voltage source; first input means for applyinga first input signal to the base of said sixth transistor; second inputmeans for applying a second input signal to the base of said secondtransistor; a first constant current circuit connected between theemitter of said first transistor and ground; a second constant currentcircuit connected between the emitter of said second transistor andground; a third constant current circuit connected between the collectorof said first transistor and said first voltage source; and a fourthconstant current circuit connected between the collector of said secondtransistor and said first voltage source.